1. Field of the Invention
This invention relates to a method of performing ultrasonic measurement, and to an apparatus therefor.
2. Description of the Prior Art
One of the systems for ultrasonic measurement in which major advances have been made in recent years is an ultrasound scanner system for medical diagnosis. An apparatus of this type which has come into practical use operates by utilizing a so-called "pulse echo" principle. An ultrasonic pulse is transmitted into a living body and is reflected at a point where the in vivo acoustic impedence is discontinuous. The reflected pulse, namely an echo of the transmitted pulse, is received in the form of an echo signal attenuated to a certain degree owing to the ultrasonic propagation through the living body. The amount of attentuation is corrected by an STC (sensitivity time control) circuit, after which the value of the echo amplitude is subjected to luminance modulation and displayed in the form of a tomograph on a cathode-ray tube by a so-called "B-mode" method. Though the echo signal contains such information as in vivo ultrasonic attenuation and in vivo propagation velocity of sound in addition to the information relating to acoustic impedence, with the B-mode method the in vivo propagation velocity of sound is assumed to be constant and attenuation is corrected for in an arbitrary manner. Consequently, the tomograph obtained is a qualitatively imaged two-dimensional distribution of the in vivo acoustic impedence interface, so that the morphological information relating to the position and shape of biological tissue inevitably forms the core of the information utilized. The state of the art is such that such biological characteristics as degree of attenuation and propagation velocity of sound are not measured, thus making it difficult to perform diversified diagnosis such as, for example, functional diagnosis.
Attempts as measuring propagation velocity of sound in biological tissue by ultrasonic computed tomography (CT) using a transmission method have been reported. See for example the "Japanese Journal of Medical Ultrasonics", vol. 7, No. 1, 1980, pp. 35-44, written by Choi Jong-Soo, and Shinichi Matsubara et al., and "Image Processing for Medical Engineering", edited by Morio Onoe. The principle involved here may be understood from FIG. 1. An ultrasonic probe (i.e. ultrasonic transducer) 10 for transmission emits an ultrasonic pulse 12 which passes through a object (living body) 14 and is received by an ultrasonic probe 16 for reception. The period of time from transmission to reception is measured by utilizing the so-called TOF (time of flight) principle to obtain projection data across the object 14 in a direction parallel to a certain axis. As in X-ray computed tomography, projection data from many different angles are collected over an angular range of 180.degree. relative to the object and the distribution of in vivo propagation velocity of sound is calculated by using a reconstruction algorithm such as a filtered back projection, which is well-known in the art. However, application of this particular method is limited to regions such as the human breast where the ultrasonic waves are capable of being transmitted through the living body over the range of 180.degree.. Application to other regions which include bone or air is not possible in actual practice.
In order to do away with this limitation upon the scope of application, ultrasonic measurement has been attempted by making use of the aforementioned pulse echo prinicple rather than the transmission principle. In a typical arrangement, an ultrasonic pulse is transmitted into a living object from two different directions and a tomograph based on the echo signals is displayed for each direction by the B-mode method. Utilizing the fact that two images may be observed with some shift between them resulting from a refraction phenomenon ascribable to a disparity in propagation velocity of sound, this arrangement attempts to measure the propagation velocity of sound based on the amount of shift that results from a blood vessel or other known object within the object. However, this scheme suffers from major drawbacks such as the need for a known object to be present within the object and the fact that the amount of shift cannot be found unless assumptions are made concerning propagation velocity of sound. The set-up therefore does not truly provide an approach satisfactory for widening application with respect to the human body.
Other methods include a scheme for finding propagation velocity of sound using a cross-beam technique. In this connection, see the Fifth International Symposium on Ultrasonic Imaging and Tissue Characterization, "Ultrasonic Imaging", vol. 5, No. 2, April 1983, p. 168. The theory involved will be described with reference to FIG. 2. The set-up includes a ultrasonic probe 18 for transmission directed at the object 14, and two ultrasonic probes 20, 22 for reception arranged at predetermined locations where reflected waves are received in parallel, these directions being different from that in which ultrasonic waves are transmitted by the probe 18. The probe 18 emits an ultrasonic pulse 24 which is scattered by a scatterer 26 inside the object 14. The scattered ultrasonic wave is then received by the ultrasonic probe 20, with the time from pulse emission to reception being designated T.sub.18.fwdarw.20. This period of time is measured. Likewise, an ultrasonic pulse 24 emitted by the probe 18 is scattered by a second scatterer 28 inside the object 14 and the scattered ultrasonic wave is then received by the ultrasonic probe 22, with the time from pulse emission to reception being designated T.sub.18.fwdarw.22. This period of time is also measured. This is followed by computing the difference between the time periods T.sub.18.fwdarw.20, T.sub.18.fwdarw.22, on the basis of which the propagation velocity of sound between the scatterers 26, 28 is found from the following equation: EQU T.sub.18.fwdarw.20 =(x.sub.1 /c.sub.1)+(x.sub.2 /c.sub.2) EQU T.sub.18.fwdarw.22 =(x.sub.1 /c.sub.1)+(l/c)+(x.sub.3 /c.sub.3) EQU T.sub.18.fwdarw.22 -T.sub.18.fwdarw.20 =(l/c)+(x.sub.3 /c.sub.3)-(x.sub.2 /c.sub.2) (1)
where x.sub.1, x.sub.2, x.sub.3, l respectively denote the distance between the ultrasonic probe 18 and the scatterer 26, the distance between the ultrasonic probe 20 and the scatterer 26, the distance between the ultrasonic probe 22 and the scatterer 28, and the distance between the scatterers 26, 28, and c.sub.1, c.sub.2, c.sub.3, c represent the mean propagation velocities of sound across the respective distances. In Eq. (1), l is capable of being measured in advance as the distance between the ultrasonic probes 20, 22 because of the abovementioned parallel conditions under which these probes are arranged. If (x.sub.3 /c.sub.3)-(x.sub.2 /c.sub.2) on the right side of Eq. (1) is assumed to be zero, then the propagation velocity c of sound may be found from c=l/(T.sub.18.fwdarw.22 -T.sub.18.fwdarw.20). However, the assumption holds good only when x.sub.3 /c.sub.3 =x.sub.2 /c.sub.2 holds. This means that even if the set-up is such that, e.g., x.sub.3 =x.sub.2 is established, it will still be necessary to impose the requirement c.sub.3 =c.sub.2, which runs counter to the intended purpose of actually measuring the propagation velocity of sound. Though the foregoing method is effective in a situation as shown in FIG. 3, in which the propagation velocity of sound of a portion contained in a thin object 32 immersed in a known medium 30 is measured in vitro, it goes without saying that the method is theoretically inapplicable to in vivo situations for the reasons set forth above.